Support metadatagroup in object

[gyscos]

This changes the model to support cases where an <object> tag includes a <metadatagroup> in addition to a <mesh>.

Here's an example of this situation from the official 3mf specs.

ObjectData can no longer be an enum, and needs to be merged into its Object parent.

The result is a breaking change.

[gyscos]

Fixes #53

[hannobraun]

Thank you for this pull request, @gyscos!

I haven't been using 3mf-rs for importing so far, and my understanding of the spec is also limited. But this looks all very reasonable, so I'm happy to merge.

[gyscos]

Thanks a lot! I can imagine fornjot mostly exports 3mf files (though being able to import existing geometry might still be useful!).

Any idea when you'll have time to release 0.6.0?

[hannobraun]

Thanks a lot! I can imagine fornjot mostly exports 3mf files (though being able to import existing geometry might still be useful!).

Yeah, importing geometry is going to happen at some point, but so far Fornjot is just exporting.

Any idea when you'll have time to release 0.6.0?

Hopefully next week, if nothing unexpected gets in the way. Let me know, if it's urgent for you. I can probably speed it up a little.

[gyscos]

Nothing urgent - we're waiting for this in stl-thumb, but nothing's on fire :)

[hannobraun]

@gyscos
New version is released: #55